Trickle beds are a widely used class of three-phase chemical reactors which consist of a bed of solids, usually a catalyst, over which liquid and gas streams flow. The flow is usually downwards and concurrent, but countercurrent applications are occasionally encountered in which a gas phase is forced upwards against a descending liquid. This class of reactor finds application to many types of reaction systems. In some, one reactant is a gas and the other is a liquid under normal operating conditions. In others, the liquid phase simply serves to remove heat or the products of the reaction and all reactants enter the system in the gas phase. There are also a few cases in which the reactants enter in the liquids but one or more of the products are swept out by the gas phase.
Examples of the first type of such systems are hydrodesulphurization of refinery process streams, such as naphtha, gas oil, lube stocks, residuum, kerosene, jet fuel, etc.; hydrocracking of heavy gas oils and residuum; hydrodemetallization of gas oils and residuum; hydrogenation of edible oils and fats; tall oil hydrogenation and many other hydrogenation reactions which are part of chemical syntheses. Highly exothermic halogenation and oxidation reactions are also sometimes carried out in trickle beds.
Aqueous reactions also are carried out in this type of reactor, including oxidation of phenolic waste streams and other soluble pollutants; as are various biochemical oxidation reactions.
As noted above, a second use of trickle bed reactors is for highly exothermic reactions between gaseous reactants where the liquid phase serves as a heat sink or as a scavenger for reaction products. Examples of the former reaction are dimerization reactions, such as butylene to octanes, and the Fischer-Tropsch synthesis; while examples of the latter reaction are sulphur dioxide oxidation over activated carbon catalysts.
One common feature of all the reactions carried out in trickle gas reactors is that the gaseous reactant(s) must diffuse through the liquid to reach the catalyst surface. The liquid in the bed is either caught in pockets and is more or less stagnant or moves as a film across the particle (catalyst) surface. In the former condition, the liquid does not effectively participate in the reaction, whereas, in the latter one, the liquid provides a barrier or a resistance for the transport of the gaseous reactant(s). This barrier or resistance, in some cases, can lower the rate of reaction and thereby decrease conversion. It is not possible to eliminate the liquid phase entirely as a means of increasing conversion because its presence is essential to the system.
Conventional trickle bed design and operation recognize that there is a minimum liquid flow rate which must be maintained to completely wet the catalyst. If this minimum is not met and the reaction is exothermic, hot spots in the bed can develop through higher rates at points where the liquid barrier is no longer present. The heat so released dries out the catalyst further, increasing the rate and leading thus to the hot spot and destabilization of the trickle bed. The hot spot problem is well documented (Gianetto & Silveston, "Multiphase Reactors Theory, design and Scaleup", Hemisphere Press, 1986) and it is accepted that, for satisfactory operation, liquid flow rates must be set to be above the minimum for the reactor system.